Etching printing plates



Dec. 8, 1959 L. H. DORO ETCHING PRINTING PLATES 2 Sheets-Sheet 1 Filed Oct. 31, 1955 INVENTOR.

LESTER H. DORO BY $0M ATTORNEYS Dec. 8, 1959 L. H. DORO ETCHING PRINTING PLATES 2 Sheets-Sheet 2 Filed Oct. 51, 1955 CLEARWATER L FIG.

TO SEWER INVENTOR.

LESTER H. DORO ATTORNEYS United States Patent ETCHING PRINTING PLATES Lester H. Doro, Appleton, Wis., assignor to Doro Corporation, Appleton, Wis., a corporation of Wisconsin Application October 31, 1955, Serial No. 543,941

3 Claims. (Cl. 41-42) The invention relates to improvements in etching plates and is of particular value for the production of plates used for relief printing or as dies used in making printing plates or other printing devices. It has value for half-tone work as well as for line work.

One of the problems encountered in the etching of printing plates is the large amount of time involved in the etching operation (meaning the period or periods during which the acid is removing the undesired metal from between the printing areas). Another problem is that in most etching operations the etching has to be carefully watched and controlled by a person highly skilled in the art of etching or engraving printing plates, and must be accomplished in successive bites separated by powdering steps. Another difi'lculty resides in the fact that in the ordinary plate etching operation, in spite of the utmost skill and care used by the operator, there is always the danger of excessive undercutting or overhang which causes cleaning difficulties and also trouble in the preparation of stereotype plates by the use of a mat or matrix, as in newspaper work.

The object of the present invention is to provide a method by which the etching of the plate can be eifected in a single bite, in a matter of a few minutes or less, in a simple and efficient manner to the required depth without the use of highly skilled, high-priced operators.

A further object is to produce, at low cost, a printing plate having lands with sloping edges so that it may be used efiiciently for newspaper or stereotype work and, in general, to provide an improved process for producing an etched plate of the character indicated.

Although other types of apparatus no doubt may be employed within the orbit of my invention, I have illustrated in the accompanying drawings one type of apparatus with which I have obtained excellent results in practicing my process. In these drawings:

Fig. 1 is a side elevation, partly in section, of the etching tank and associated equipment;

Fig. 2 is an end elevation of the apparatus looking in the direction of the arrows 2 in Fig. 1, certain parts being shown in section;

Fig. 3 is a plan view of the apparatus shown in Fig. 1;

Fig. 4 is a perspective view, greatly enlarged, showing a portion of a printing plate made by my improved process with the apparatus disclosed in the drawings; and

Fig. 5 is a similar view of a modification.

Referring to the drawings, the numeral represents a tank made of ceramic ware or other acid-proof material and in which the etching process is effected in a bath of hydrochloric acid 11 in which the magnesium plate or plates 12 are fully immersed during the actual etching operation. The plate or plates 12 are carried on the face of a disc or spider 13 which is at the end of a shaft 14 mounted to rotate in the lower end of a leg 15. The upper end of the leg 15 is bolted to one end of the arm 16, the other end of the arm 16 being connected by a hinge 17 to a block 18 carried by a turntable 19 rotatably mounted upon a stud 20 fixedly mounted in the top of ice a bench or table 21. By means of the hinge 17 the arm 16 may be raised into the dotted line position shown in Fig. 1, and having been so raised so as to clear the upper edge of the tank 10, the arm may be swung on pivot 20 into positions registering with the supplemental tanks 22 and 23 illustrated in Fig. 3.

The first wash tank 22 is lightly contaminated with acid when the plate is withdrawn from the tank 10 and dunked in the tank 22, and this tank 22 receives wash water from the tank 23 which is fed with clear water through a valve 24, and into which tank 23 the final clear water washing of the plate is effected.

In view of the fact that the etching operation results in the evolution of a pungent and irritating gas, the upper end of the tank 10 is fitted with a cover 25. This cover 25 is fastened to a leg 15 by a bracket 26, so that when the arm 16 is raised the cover 25 is also raised in order to permit the printing plate to be withdrawn.

An important feature of the invention resides in the use of a positive relative movement between the etching fluid, which can be hydrochloric acid, and the metal surface of the plate, in this instance magnesium, during the etching operation, so that in effect a gentle flow of the acid across the plate surface is obtained. Furthermore, I find that in order to prevent an uneven etching as a result of a one-way relative movement of the plate and acid, I alternate the movement in different directions. This may conveniently be eifected by rotating the plate 12 on its spindle 14. This is done by securing on the end of the spindle 14' a grooved pulley 27 which accommodates a rubber belt 28 passing around a similar grooved pulley 29 fixed upon the end of a spindle 30 adapted to rotate in the extended upper end 31 of the leg 15. The other end of the shaft or axle 30 is provided with a hub 32 and an arm 33 fixed thereon, the outer end of said arm being provided with a handle 34 by means of which the shaft 30 may be rotated in either direction.

The cover 25 fits closely around the leg 15 and is provided with slots or openings through which the belt 28 can freely pass. Such gases as are formed in the etching tank 10 during the etching operation are withdrawn through an exhaust pipe 35 which preferably is connected to an exhaust fan. It will be understood that all parts of the apparatus which come in contact with the acid or the exhaust gases are made of non-corrosive material or are coated with a non-corrosive material.

In practicing the process, the preparation of the plate for etching is not greatly different from that employed in customary photo-engraving processes. The bare surface of the side of the plate which is to be used for printing is coated with a light-sensitive resist. Any conventional resist may be used. However, I prefer to use a commercial photo-engraving resist, preferably having a somewhat greater flexibility than conventional bichromated shellac.

The resist-coated plate is then exposed to light which is passed through a photographic negative of the subject. The light striking the resist coating on the plate insolubilizes the resist areas which in the subject will be represented as dark areas and which in the negative are clear. The areas of the resist which are not subjected to light are not insolubilized and may be removed by a simple washing process, leaving the resist coating on only those areas of the subject which are to stand in relief in the finished plate and receive the ink when used in the printing press.

I have discovered that from the standpoint of obtaining the best results, the areas of resist which remain on the plate prior to the etching operation should be larger than the corresponding areas of the finished printing plate. This is particularly important in the case of the small dots in the high-light areas of the subject. A skilled cameraman has no difficulty in enlarging these high-light dot areas of the resist by suitably manipulating his camera setup in the production of the original negative.

In the case of line work, the lines on the original drawing may be enlarged by hand.

After the plate 12 has been treated as above described, it is secured to the rotary spider 13 of the apparatus and then the arm 16 is promptly lowered so as to completely immerse the plate 12 in the etching acid 11. This acid may be a good commercial grade of hydrochloric acid diluted to a strength of from 1 Baum to 11 Baum, depending upon the temperature of the bath, and preferably containing about three and one-half ounces (dry weight) of papermakers alum (aluminum sulphate) for retarding and controlling the action of the acid, except for half-tone work where the alum is not necessary. A weak acid will normally require a higher temperature, while a stronger acid should be employed in the case of a very cold acid solution. Although a range of temperatures from 40 F. to 105 F. is feasible, in accordance with the concentration of the solution, it is preferred to use the bath at a normal room temperature of from about 70 F. up to about 80 F., and at a concentration of about 4 to 6 Baum. Such range of concentrations and temperatures will permit the etching process to be eltected in a brief period of between 30 seconds and two or three minutes, depending upon the character of the work, while the etching action will not be so violent as to be unmanageable.

In the normal operation, in making a half-tone printing plate for newspaper work, and using a 65-line screen, with an etching plate of about .006 inch in the highlights, the period of immersion in the tank is about 45 seconds. During the period of immersion, the plate is rotated in the bath by means of the crank 34 in opposite directions at about 35 r.p.rn. with a reversal of rotation about once every seconds. After the etching operation in the tank 10 is completed, the arm 16 is raised and swung to a position above the tank 22 and the plate is immediately dunked into the wash water in that tank while still rotating the plate, so as to eliminate most of the relatively strong acid picked up on the equipment from its contact with the strong acid in the tank 10.

After the first wash in tank 22, the apparatus is given a similar second wash in the clear water tank 23, after which the plate can be removed from the spider. It may be advantageous in some cases to clean up the plate by dipping the plate for a few seconds in a bath of dilute nitric acid having a concentration of not greater than 5%. Then the plate can be washed off and dried, and is ready for printing.

I am unable to supply definite proof of what precisely occurs during the etching operation when carried out according to my process. I believe that the tapered effect of the edges of the printing areas may be due to the fact that the marginal edges of the resist areas, possibly due to the development of hydrogen gas at the edge of the metal, or possibly due to the movement of the acid flowing past the edge of the resist, bend or fold away from the metal, allowing the acid to float past the edge of the metal at a faster rate than in the recesses, thus accelerating the etching effect at the top of the metal as compared with the etching efiect lower down in the recess of the plate.

In Fig. 4, which shows an enlargement of a portion of a line engraving, it will be seen that the edges of the printing areas or lands of the plate have the shape of fillets and exhibit no overhang. This fillet shape which is so greatly desired by printers is a characteristic of a printing plate produced by my improved process and apparatus.

In Fig. 5, which shows another line engraving 12A made by my general process, it will be seen that at the base of the land below the fillet, and around the land, there is a tiny moat or groove 36 which is of considerable value in certain special types of plates, for example in the manufacture of dies for dandy rolls used for applying watermarks in papermaking. I have discovered that this groove effect may be efficiently produced by using sulfuric acid alone as the etching fluid. The sulfuric acid is preferably used at a concentration of from about 14 to about 20 Baum at a temperature of about 80 F.

The term printing plate used in this specification, is intended to include any kind of plate used either directly or indirectly as a means for making another plate or other device by which the final printed result is obtained.

I claim:

I. The process of etching a magnesium plate for use in relief printing which comprises (1) treating the bare surface of the printing side of the plate by (a) coating it with a soluble resist, (b) exposing said coating to the action of light controlled by a photographic negative of the subject so as to insolubilize those areas of the coating which are intended to represent the printing areas of the plate, (0) washing out other areas of the coating, and (2) (a) completely immersing the treated plate in a bath of hydrochloric acid, (b) during the immersion period rotating the immersed plate in the bath alternately in opposite directions, (c) promptly removing the plate from the bath when the plate has been sufiiciently etched, and (d) then promptly washing the plate.

2. The process of etching a magnesium plate to be used for relief printing which comprises (1) treating the bare surface of the printing side of the plate by (a) coating it with a soluble resist, (b) exposing said coating to the action of light controlled by a photographic negative of the subject so as to insolubilize those areas of the coating which are intended to represent the printing areas of the plate, (0) washing out other areas of the coating, and (2) (a) completely immersing the treated plate in a bath of hydrochloric acid at a concentration and temperature of from about 1 Baurn at a temperature of about 105 F. to about 9.5 Baum at a temperature of about 40 F., (b) during the immersion period effecting a relative movement between the treated surface of the plate and the liquid in the bath and relatively rapidly alternating the direction of said movement, (0) promptly removing the plate from the bath when the plate has been sufficiently etched, and (d) then promptly washing the plate.

3. The process of etching a magnesium plate to be used for relief printing which comprises (1) treating the bare surface of the printing side of the plate by (a) coating it with a soluble resist, (b) exposing said coating to the action of light controlled by a photographic negative of the subject so as to insolubilize those areas of the coating which are intended to represent the printing areas of the plate, (a) washing out other areas of the coating, and (2)(a) completely immersing the treated plate in a bath of hydrochloric acid at a concentration of 4 to 6 Baum and at a temperature of between F. and F., (b) during the immersion period effecting a relative movement between the treated surface of the plate and the liquid in the bath and relatively rapidly alternating the direction of said movement, (0) promptly removing the plate from the bath when the plate has been sufiiciently etched, and (0.) then promptly washing the plate.

References Cited in the file of this patent UNITED STATES PATENTS 1,330,581 Coulson Feb. 10, 1920 1,918,545 Hoy July 18, 1933 2,399,534 Alger Apr. 30, 1946 2,801,909 Hirdler Aug. 6, 1957 

1. THE PROCESS OF ETCHING A MAGNESIUM PLATE FOR USE IN RELIEF PRINTING WHICH COMPRISES (1) TREATING THE BARE SURFACE OF THE PRINTING SIDE OF THE PLATE BY (A) COATING IT WITH A SOLUBLE RESIST, (B) EXPOSING SAID COATING TO THE ACTION OF LIGHT CONTROLLED BY A PHOTOGRAPHIC NEGATIVE OF THE SUBJECT SO AS TO INSOLUBILIZE THOSE AREAS OF THE COATING WHICH ARE INTENDED TO REPRESENT THE PRINTING AREAS OF THE PLATE, (C) WASHING OUT OTHER AREAS OF THE COATING, AND (2) (A) COMPLETELY IMMERSING THE TREATED PLATE IN A BATH OF HYDROCHLORIC ACID, (B) DURING THE IMMERSION PERIOD ROTATING THE IMMERSED PLATE IN THE BATH ALTERNATELY IN OPPOSITE DIRECTIONS, (C) PROMPTLY REMOVING THE PLATE FORM THE BATH WHEN THE PLAET HAS BEEN SUFFICIENTLY ETCHED, AND (D) THEN PROMPTLY WASHING THE PLATE. 